Semiconductor fabrication includes a plurality of process steps (e.g., furnace operations such as thermal oxidation or low pressure chemical vapor deposition (LPCVD) that involve processing a plurality of different production lots of product wafers together in one “batch” process. A production wafer lot may include 25 wafers, and a batch may include 6 lots so that each batch for batch processing includes 150 wafers, along with a monitor lot including one or more monitor wafer(s) that are generally unpatterned wafers having minimal (if any) earlier processing to represent the product wafers to allow measurement of the layer formed in the batch process which is not easily obtainable (and sometimes not possible) from the product wafers due to layers and/or topography formed on the substrate from earlier processing.
Each production lot in a batch is staged with the monitor lot to move through the same steps including pre-measurements step(s) (e.g., pre-particle data and pre-thickness data) in order for the lots to receive the needed process control data at the batch process step, such as the thickness of a deposited layer (e.g., silicon nitride). Monitor units constituting a monitor lot are thus needed for each lot to accommodate for the different process conditions for each production flow. All lots must generally move together which delays staging and process time while the initial (pre) process control measurements are made.